I personally haven't been seeing a lot of problems with caster champions myself. Aside from occasionally the stars aligning for them and being forced to run in circles jumping with a lootgen mace to bypass DR avoiding randomized buffs that give more than 40 hp lightning bolts, they are fairly easy to handle in heroics. Main issues I've seen is spawn rate, and the buffing of melee toons. When a caster throws out a lightning bolt once every 4-5 seconds, that gives you time to heal. Having enemies like kobolds that can already hit fairly quickly all of a sudden have DR that makes pointless or reduced your main damage, doubled or more health at low levels, extra damage+extra damage weapon effect, and potentially fort bypass/true seeing can be very painful; especially when it's five kobolds at once.
I've had less problems in groups than solo by far with these effects, but still a lot of problems. The main issue with this change that I have in that regard is that increasing the difficulty in this way (i.e. easier to have random/low control deaths) forces people to group for most questing; which would be completely fine were this WoW. The player base in Dungeons and Dragons just isn't high enough to support group play at all levels for the content that you need though, due primarily to prior changes driving away players, therefore the actual effect of this system will be for anyone that doesn't want TR time doubled or more and resources consumed much faster to go find another game.
Ran a level 3 alt with 11 heroic past lives through some harbor quests. Was by no means a gimp toon, and had about 20 PRR at that level. Multiple times wiped because I'd run into an enemy that was reducing my damage to the point of almost not being able to damage, taking several moderate damage attacks, and then taking an extra damage critical attack with force/neg procs for 60+10 damage. In higher heroic and epic levels, this problem is staying the same because of fort being negated in many situations by champion buffs combined with passive monster fort bypass. Took hits in many EE quests for 700-1400 damage on my 30+ life main character in epics with 190% fort and 140 PRR from enemies that either score a 3x-4x critical, or score an extra damage hit that bypasses PRR.
Also, just think you might want to question the logic of testing some of the highest damage numbers people have been seeing with the lowest content.